A typical pulse combustor (or pulsejet engine of the valveless type) consists of a combustion chamber, an inlet pipe, fuel injector(s), spark plug (or other ignition means), and an exhaust pipe (sometimes referred to as ‘tailpipe’). The combustion chamber, inlet pipe and exhaust are often cylindrical. The diameter of the inlet and exhaust pipes is typically less than the diameter of the combustion chamber, and the length of the inlet pipe is typically smaller than the length of the exhaust pipe.
When fuel and air are introduced into the combustion chamber, a spark produced by the spark plug or other ignition means ignites the fuel-air mixture. The ensuing combustion process causes a rise in the temperature and pressure of the gases inside the combustion chamber. These gases then expand and escape through the inlet and exhaust pipes. The high velocity of the escaping gases causes an overexpansion and negative pressure inside the combustion chamber. This negative pressure then reverses the direction of the flow in the inlet and exhaust pipes. Fresh air sucked in from the atmosphere via the inlet pipe mixes with the fuel (which is injected either in the inlet pipe or directly into the combustion chamber) and enters the combustion chamber where it encounters high-temperature combustion products from the previous combustion event. These combustion products ignite the fresh fuel-air mixture to produce another combustion event and the process repeats indefinitely. It should be noted that there is also flow reversal in the exhaust pipe due to the negative pressure in the combustion chamber, but due to the longer length of the exhaust pipe, the fresh air drawn in from the atmosphere does not typically reach the combustion chamber before the process restarts. It should also be noted that the spark plug is only needed to start operation of the engine, and is not necessary to sustain the operation of the engine. Therefore, the spark plug can be turned off once the engine has started.
The net result of the working cycle of a pulse combustor is that the inlet and exhaust ends produce oscillating flows, i.e. intermittent jets, of gas which are responsible for thrust generation. The exhaust pipe usually generates the highest amount of thrust, but the inlet pipe can also generate a significant amount of thrust. Therefore, in order to make the thrust from the inlet pipe point in the same direction as the thrust from the exhaust pipe, the inlet pipe is often turned to point in the same direction as the exhaust pipe (or vice versa), giving the engines a “U-shape.” Pulse combustors can be made in a variety of forms. Some have multiple inlets, while others have inlets that are perpendicular to the exhaust pipe. Nevertheless, all these embodiments have similar working principle and are essentially the same device.
This apparatus can be used as a combustor. The advantages of pulse combustors include the ability draw in fresh air and sustain operation without any external machinery or moving parts. Pulse combustors can also be used as thrust-producing devices, in which case they are commonly referred to as “pulsejet” or “pulse jet” engines. Pulsejet engines have a long history and have been used to propel several aircraft over the last century. They are often characterized by a diverging exhaust pipe to aid in thrust production.
Pulsejet engines are characterized by their simplicity because of the lack of moving parts. However, the oscillating nature of the exhaust flows tends to produce very high noise and vibration levels that have often been cited as the most serious hurdle in their widespread implementation. The U-shape of many pulsejet engines can also pose a challenge for airframe integration. This U-shape configuration also entails that the inlet and exhaust pipes are both facing (opening) away (opposite) from the incoming airstream in flight, which disables the engine from using the potentially-advantageous ram air pressure of the incoming airstream, thereby limiting its high-speed performance.
The invention described below is intended to address the above issues and to provide related advantages.